Distributed array for direction and frequency finding

ABSTRACT

An optical imaging system and method that reconstructs RF sources in k-space by utilizing interference amongst modulated optical beams. The system and method involves recording with photodetectors the interference pattern produced by RF-modulated optical beams conveyed by optical fibers having unequal lengths. The photodetectors record the interference, and computational analysis using known tomography reconstruction methods is performed to reconstruct the RF sources in k-space.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.15/227,859 filed Aug. 3, 2016 which claims the benefit of U.S.Provisional Application No. 62/200,626 filed on Aug. 3, 2015, thedisclosure of each of these applications hereby being incorporated byreference in its entirety.

FIELD OF TECHNOLOGY

The herein described subject matter and associated exemplaryimplementations are directed to improvements and extensions of animaging receiver as described in U.S. Pat. No. 7,965,435 and US PatentPublication No. 2016/0006516, the disclosure of each being herebyincorporated by reference in its entirety.

BACKGROUND

Many existing antenna arrays are unable to detect both location andfrequency of an incoming RF signal without significant filtering orother processing. In such systems, the received broadband radiation isdivided into multiple narrow-band channels that are processedindividually to determine the information content, and, potentially, theangle of arrival (AoA) of the received radiation. Such processingrequires banks of high-speed receivers to sift through the vast amountof data in search of signals of interest. Imaging receivers may rely ondistributed aperture to sample incoming electromagnetic radiation, whichis then up-converted to optical domain for conveyance and processing.The up-conversion process preserves the phase and amplitude informationof radio frequency (RF) waves in the optical domain, which therebyallows optical reconstruction of the RF scene. However, the opticalreconstruction in imaging receivers (the spatial location of the opticalsignals on the image sensor) is dependent on the frequency of the RFwaves. Thus, when there are sources of different RF frequency beingprocessed simultaneously, their locations in the real world could not bepreviously unambiguously identified by imaging receivers.

SUMMARY

The herein described exemplary implementations provide novel approachesto extracting information about radio frequency (RF) emitters fromreceived electromagnetic radiation ranging between 100 MHz and 300 GHz.The exemplary implementations may provide real-time, simultaneousdetermination of carrier frequency, amplitude and angle of arrival(AoA). In some exemplary embodiments, instantaneous bandwidth (IBW) mayapproach 100 GHz. This capability may be achieved without sacrifice ofsignal-to-noise ratio (SNR), by virtue of an antenna array whose gainmore than compensates for the added thermal noise that accompanies suchwide IBW. The optical approach may enable the array's entire field ofregard (i.e. its full beam steering range) to be continuously detectedand processed in real time.

One exemplary implementation of an optical imaging receiver includes aphased array antenna having a plurality of antenna elements arranged ina first pattern configured to receive RF signals from at least one RFsource and a plurality of electro-optic modulators corresponding to theplurality of antenna elements, each modulator configured to modulate anoptical carrier with a received RF signal to generate a plurality ofmodulated optical signals. The imaging receiver further includes aplurality of optical channels configured to carry the plurality ofmodulated optical signals and configured to cause interference amongstthe optical signals, each of the plurality of optical channels having anoutput to emanate the corresponding modulated optical signal out of thecorresponding optical channel. The outputs of the plurality of opticalchannels being arranged in a second pattern which does not correspond tothe first pattern of the antenna array. A plurality of photodetectorsfor recording the optical signal interference and a module forcomputationally reconstructing RF sources in k-space from the recordedinterference are also included.

Another exemplary implementation of an optical imaging receiver includesa phased-array antenna including a plurality of antenna elementsarranged in a first pattern configured to receive RF signals from atleast one RF source and a plurality of electro-optical modulatorscorresponding to the plurality of antenna elements, each modulatorconfigured to modulate an optical carrier with a received RF signal togenerate a plurality of modulated optical signals. Also included is aplurality of optical fibers arranged in a second pattern whichcorresponds to the first pattern of the antenna array, with each of theplurality of optical fibers having varying lengths, and a plurality ofphotodetectors for recording optical signal interference occurring infree space after the optical signals are released from their respectiveoptical fibers. A module is provided for computationally reconstructingRF sources in k-space from the recorded interference.

An exemplary implementation of an imaging method utilized by an imagingreceiver includes receiving incoming RF signals at a phased-arrayantenna including a plurality of antenna elements arranged in a firstpattern and modulating the received RF signals from each of theplurality of antenna elements onto an optical carrier to generate aplurality of modulated optical signals. The method also includesdirecting the plurality of modulated optical signals to a plurality ofoptical channels, configured to cause interference amongst the opticalsignals. Each of the plurality of optical channels having an output toemanate the corresponding modulated optical signal out of thecorresponding optical channel, the outputs of the plurality of opticalchannels being arranged in a second pattern which does not correspond tothe first pattern of the antenna array. The method further includesproviding a plurality of photodetectors for recording the optical signalinterference and computationally reconstructing RF sources in k-spacefrom the recorded interference.

Another exemplary implementation of an imaging method utilized by animaging receiver includes receiving incoming RF signals at aphased-array antenna including a plurality of antenna elements arrangedin a first pattern and modulating the received RF signals from each ofthe plurality of antenna elements onto an optical carrier to generate aplurality of modulated optical signals. The method further includesusing a plurality of optical fibers, arranged in a second pattern whichcorresponds to the first pattern of the antenna array, the opticalfibers having varying lengths. The method further includes providing aplurality of photodetectors for recording optical signal interferenceoccurring in free space after the optical signals are released fromtheir respective optical fibers and computationally reconstructing RFsources in k-space from the recorded interference.

Another exemplary implementation comprises an antenna array comprising aplurality of antenna elements arranged in a first pattern configured toreceive RF radiation; a plurality of electro-optic modulatorscorresponding to the plurality of antenna elements, each modulatorconfigured to modulate an optical carrier with the received RF radiationto generate a plurality of modulated optical signals; a plurality ofoptical channels configured to carry the plurality of modulated opticalsignals; a plurality of optical phase-adjustment means corresponding tothe plurality of the optical channels; a plurality of optical-channeloutputs arranged in a second pattern wherein the second pattern is ascaled and substantially planarized version of the first pattern; meansfor effecting substantially unequal time delays between a wave-front ofthe received RF radiation and the optical-channel outputs for at leastsome of the different optical channels; a compound optical channelcoupled to the plurality of optical-channel outputs and configured tocause interference amongst the modulated optical signals; and aplurality of photodetectors for recording the optical beam interference;wherein the plurality of optical-phase adjustment means may be adjustedto substantially cancel the unequal RF phase delays incurred due to theunequal time delays at a selected frequency of the RF radiation.

The plurality of optical channels may comprise a plurality of opticalfibers; and the means for effecting unequal time delays may compriseunequal lengths of the optical fibers, or true-time-delay elements.

The means for effecting unequal time delays may comprise having theelements of the first pattern non-coplanar.

The system may also comprise means to substantially suppress received RFradiation at frequencies substantially differing from the selectedfrequency.

The plurality of photodetectors may form an array and/or comprise animage sensor.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is an illustration of an RF receiver in accordance with aspectsof the invention;

FIG. 1B is another illustration of an RF receiver in accordance withaspects of the invention;

FIGS. 2A, 2B and 2C are a block diagrams of components for use with theRF receiver of FIG. 1A or 1B;

FIG. 2D illustrates details of an embodiment where the detector maycapture in real-time an image of a selected frequency of an RF scene;

FIGS. 3A, 3B and 3C are schematic drawings illustrating various RF planewaves detected by the imaging receiver of FIG. 1;

FIG. 4 is a schematic drawing illustrating k-space representation of theimaging receiver;

FIG. 5 is a schematic drawing illustrating of a generalized imagingreceiver;

FIG. 6 is a graphical representation of RF scene reconstruction;

FIG. 7 depicts the sampling of the spatial-temporal aperture used in thereconstruction of FIG. 6;

FIG. 8 is a schematic drawing illustrating an imaging receiver arrangedso as to receive off-axis incidence for all received RF radiation;

FIG. 9 depicts the projection of k-space in the receiver configurationof FIG. 8;

FIG. 10 is a schematic drawing illustrating a combination of twooff-axis imaging receivers for AoA/frequency disambiguation;

FIG. 11A is a schematic drawing illustrating a visualization of 3Dk-vector space performed by two arrays;

FIG. 11B is a schematic drawing illustrating a visualization of 3D realspace performed by visual stereoscopic imaging;

FIG. 12 is a schematic drawing illustrating a dual array for 3D k-spacereconstruction with interleaved arrays;

FIG. 13 depicts k-space representation of stereoscopic imager with twosets of lines of projection;

FIG. 14 is a schematic drawing illustrating a single array with two RFimaging axes;

FIG. 15 is a schematic drawing illustrating a dual imaging receiver withshared optical reconstruction system; and

FIG. 16 is a flow chart of a method performed by an imaging receiver inaccordance with the disclosed exemplary implementations.

DETAILED DESCRIPTION

The present disclosure now will be described more fully hereinafter withreference to the accompanying drawings, in which various exemplaryimplementations are shown. The invention may, however, be embodied inmany different forms and should not be construed as limited to theexample exemplary implementations set forth herein. These exampleexemplary implementations are just that—examples—and manyimplementations and variations are possible that do not require thedetails provided herein. It should also be emphasized that thedisclosure provides details of alternative examples, but such listing ofalternatives is not exhaustive. Furthermore, any consistency of detailbetween various examples should not be interpreted as requiring suchdetail—it is impracticable to list every possible variation for everyfeature described herein. The language of the claims should bereferenced in determining the requirements of the invention.

Although the figures described herein may be referred to using languagesuch as “one exemplary implementations,” or “certain exemplaryimplementations,” these figures, and their corresponding descriptionsare not intended to be mutually exclusive from other figures ordescriptions, unless the context so indicates. Therefore, certainaspects from certain figures may be the same as certain features inother figures, and/or certain figures may be different representationsor different portions of a particular exemplary implementation.

The terminology used herein is for the purpose of describing particularexemplary implementations only and is not intended to be limiting of theinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items and maybe abbreviated as “/”.

It will be understood that the terms “comprises” and/or “comprising,” or“includes” and/or “including” when used in this specification, specifythe presence of stated features, regions, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, regions, integers, steps,operations, elements, components, and/or groups thereof.

It will be further understood that when an element is referred to asbeing “connected” or “coupled” to or “on” another element, it can bedirectly connected or coupled to or on the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element, oras “contacting” or “in contact with” another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between). Unless otherwise defined, all terms (includingtechnical and scientific terms) used herein have the same meaning ascommonly understood by one of ordinary skill in the art to which thisdisclosure belongs. It will be further understood that terms, such asthose defined in commonly used dictionaries, should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthe relevant art and/or the present application, and will not beinterpreted in an idealized or overly formal sense unless expressly sodefined herein.

As is traditional in the field of the disclosed technology, features andexemplary implementations are described, and illustrated in thedrawings, in terms of functional blocks, units and/or modules. Thoseskilled in the art will appreciate that these blocks, units and/ormodules are physically implemented by electronic (or optical) circuitssuch as logic circuits, discrete components, microprocessors, hard-wiredcircuits, memory elements, wiring connections, and the like, which maybe formed using semiconductor-based fabrication techniques or othermanufacturing technologies. In the case of the blocks, units and/ormodules being implemented by microprocessors or similar, they may beprogrammed using software (e.g., microcode) to perform various functionsdiscussed herein and may optionally be driven by firmware and/orsoftware. Alternatively, each block, unit and/or module may beimplemented by dedicated hardware, or as a combination of dedicatedhardware to perform some functions and a processor (e.g., one or moreprogrammed microprocessors and associated circuitry) to perform otherfunctions. Also, each block, unit and/or module of the exemplaryimplementations may be physically separated into two or more interactingand discrete blocks, units and/or modules without departing from thescope of the inventive concepts. Further, the blocks, units and/ormodules of the exemplary implementations may be physically combined intomore complex blocks, units and/or modules without departing from thescope of the inventive concepts.

Aspects of the disclosure are related to devices and associated methodsfor improving a wideband radio-frequency (RF) phased-array receiver. Theembodiments described here may determine a signal's angle of arrival(AoA) and frequency in real time. Aspects of the embodiments provide asignal detection mechanism wherein RF signals are upconverted byfiber-coupled electro-optic modulators driven by the antenna elements ofa phased array. The conversion results in sidebands on an opticalcarrier wave supplied by a laser. These optical sidebands aresubstantially proportional in power to the RF power incident into eachantenna element, and also preserve the phase carried by the incident RFsignal. This RF upconversion allows the optical sidebands to be used toreconstruct an image of the RF energy in the scene.

An imaging receiver 100 in accordance with aspects of the invention isdepicted in FIGS. 1A and 1B wherein similar or like elements areidentified by the same reference numerals. The illustrated imagingreceiver 100 is a phased-array receiver. The imaging receiver 100includes a processor 200 coupled to the various components within thereceiver to implement the functionality described herein. The processormay be a general purpose processor (e.g., part of a general purposecomputer, such as a PC) or dedicated processor (e.g., digital signalprocessor (DSP), FPGA (field programmable gate array)). The processormay be configured with software to control the component of the imagingreceiver 100. Variations of suitable processors for use in the imagingreceiver 100 will be understood by one of skill in the art from thedescription herein.

A phased-array antenna 110, e.g., a sparse array of M antenna elements120 arranged in a first pattern as shown in the example of FIG. 1B,receives RF signals from an external source. Various patterns of thearrangement of the M antenna elements 120 are described further herein,and may include planar arrangements, conformal arrangements conformingto a non-planar three dimensional surface (e.g., a surface of a vehicle,such as the hull of an airplane or helicopter), regularly spacedarrangements (e.g., regularly spaced in a two dimensional array) or anirregularly spaced array. While the antenna elements 120 shown in FIGS.1A and 1B are horn antennae, those of skill in the art will understandthat a variety of antenna means may be used. RF signals sampled at theantenna elements 120 are used to modulate a laser beam split M ways. Anelectro-optic (EO) modulator 130 is coupled to each of the antennaelements 120 and receives a branch of the split laser beam that it usesto convert the RF energy received at each antenna element 120 to theoptical domain. It does so by modulating the optical (carrier) beamproduced by the laser 125 (FIGS. 1A, 1B, 2A). The time-variantmodulation manifests itself in the frequency domain as a set ofsidebands flanking the original carrier frequency (or wavelength), atwhich the source laser operates, as illustrated in FIG. 2B, which isdiscussed in more detail below. As a result, the energy radiated in theRF domain appears in the optical domain as sidebands of the carrierfrequency. This up-conversion of the RF signal into optical domain maybe coherent so that all the phase and amplitude information present inRF is preserved in the optical sidebands. This property of coherencepreservation in optical up-conversion allows the recovery of theRF-signal angle of arrival using optical means.

As shown in FIG. 1B, the modulated optical beams containing the lasercarrier wavelength and the sidebands with imprinted RF signal areconveyed by optical fibers 140 to a lenslet array 150 (FIG. 1B) coupledto the outputs 141 of the fibers 140 that are arranged in a secondpattern. The second pattern may or may not mimic or correspond to thefirst pattern of the array of the RF antennas at a reduced scale.

FIG. 2C illustrates the output ends of the optical fibers 140 arrangedin a pattern which may correspond to the pattern of the antenna elements120 of FIG. 1. From the outputs 141 of the optical fibers 140 at thelenslet array 150 on, the beams propagate in free space, no longerguided by the optical fibers, and form a combined beam 160 where thelight emanating from fiber outputs interfere. While this embodimentshows conventional optical fibers 140 between the electro-opticmodulators 130 and the lenslet array 150, those of skill in the art willappreciate that other optical waveguides or channels may also or insteadbe used. Similarly, while this embodiment illustrates the use of a freespace as a channel for forming a composite optical beams 160 and 185from light emanating from the outputs of the optical fibers 140, thoseskilled in the art will appreciate that other optical channels can beused for forming a composite optical beam 160 and/or 185.

As shown in FIG. 1B, the individual beams propagate in free space fromthe outputs 141 of fibers 140 at the lenslet array 150, which allows theindividual beams to interfere with one-another where they overlap toform the combined or composite beam 160. Part of the optical compositebeam 160 is split off with a beam-splitter 165, mixed with a referencebeam 170, and sent to an array of detectors 175 (phase-compensationdetectors) in order to detect, and, if desired, allow for thecompensation of, optical phase variation originating in the individualfibers 140 due to environmental conditions such as vibrations andacoustics. An optional band-pass optical filter 180, may strip off thecarrier wavelength and allow only one of the sidebands through (see FIG.2B). The resulting overlapping beams forming a composite beam projectedonto photodetector 190, e.g., an image sensor array formed on asemiconductor chip, such as a charge coupled device (CCD) array, CMOSimage sensor array, and/or a photodiode array, an optical camera, and/orother standard image sensors. Thus, the overlapping beams form compositebeam 185 where they interfere to form a representation of the RF signalin the optical domain.

As shown in FIG. 1A, the free space optics may include optical filteringand interference module 192 and photodetector array 190 which allows thebeams emanating from the outputs of fibers 140 to interfere with eachother in free space prior to detection and recordation by photodetectorarray 190.

FIG. 2B illustrates the use of an optical filter 180 to recover orisolate an optical sideband that corresponds to a received RF signal,which may for example be a millimeter wave (MMW) signal having afrequency ω_(m). As shown in the graphs of FIG. 2B, the received RFsignal(s) from antenna element(s) 120 modulate an optical carrier signal(source) 125 operating at a frequency ω₀ (illustratively at a wavelengthbetween 1557 and 1558 nm). The output 135 of modulator 130 includes anoptical analog of the MMW signal in sidebands of the optical carrier asshown in the middle graph. The output 135 of modulator 130 istransmitted via a corresponding optical fiber 140. An optical band-passfilter 180 tuned to ω₀+ω_(m) or ω₀−ω_(m) strips off (isolates) theoptical representation of the received MMW signal(s) from the carrier.

FIG. 2A depicts the configuration of an imaging receiver 100 with anemphasis on the optical layer. The single laser source 125 is split Mways by a splitter 127 and the beams 128 are routed through modulators130 coupled to antennas 120 capturing the RF radiation. The (optical)outputs 135 of the modulators 130 are filtered to allow only a singlesideband corresponding to the captured RF radiation to pass, for exampleusing a filter 180 as described with respect to FIG. 2B. The free-spaceinterference of the optical composite beam 185 output from filter 180among the M different channels yields a pattern measured with detectors190, as discussed in more detail below.

Note that FIGS. 1 and 2A depict the filter 180 positioned in thefree-space portion of the imaging receiver 100 downstream of the lensletarray 150. In some exemplary implementations the filter is optional andis not a necessary component of the system or methodology. In yet otherimplementations, the filter can be placed anywhere between themodulators 130 and the detector 190 to enable reconstruction of theRF-source position in the optical domain. Furthermore, in some exemplaryimplementations, especially for frequencies lower than ˜5 GHz, aMach-Zehnder modulator (MZM) may be used to filter out the sidebandenergy from the optical carrier energy. Such modulators can, underappropriate bias conditions, interferometrically suppress the carrierwhile passing the (odd-order) sidebands, thereby suppressing the carrierin a frequency-independent manner. In yet other implementations, nophysical filter may be used, and the system may rely on thecomputational reconstruction to account for the presence of the opticalcarrier in the interference pattern. In yet other implementations, thephysical arrangement of the optical channels, including the antennas120, the lenslet array 150 and/or the optical fiber lengths, and/or theapplied optical phases by properly biasing modulators 130, or by othermeans, may be so organized as to produce the interference pattern of thecarrier wavelength significantly separated spatially from theinterference pattern produced by the sidebands. Other implementationsmay combine some or all of the approaches listed above.

The detector 190 of FIGS. 1A and 1B may be an array of photo-detectorssuch as those of a charged coupled device (CCD) or contact image sensoror CMOS image sensor, which in some embodiments may not be able toprocess (e.g. decode) information present in the RF signals received bythe antenna array 110 with the same performance as high-speedphotodiodes. In some exemplary implementations, to extract or recoverinformation encoded in the RF signals input by the antenna elements 120,the composite optical beam output from filter 180 is further split withadditional beam-splitters and combined with reference laser beams forheterodyne detection by a high speed photodetector (see, e.g., U.S.Patent Pub. No. 2016/0006516).

Below, further details on the optical capture of the RF scene arepresented. To capture the RF scene in the optical domain, the (optical)outputs of the modulators 130 are carried in optical fibers 140 to alenslet array 150 (FIG. 2C). The arrangement of the optical fibers 140need not mimic the spatial distribution of the corresponding antennas120 to which the optical fibers are attached. For example, a sequence ofoptical fibers along a particular direction may be different than asequence of the corresponding antennas 120 to which they are attached (asequence of these antennas 120 along a particular line or curve, e.g.).The fibers may also be split so as to produce a higher number of opticaloutput beams than the number of antennas 120. However, the arrangementof the optical fibers 140 may also mimic the spatial distribution of theantennas 120 to which they are attached. The output beams are thenallowed to interfere in free space (or other suitable channel or mediumfor forming a composite optical signal), and the interference patterncorresponding to the original RF scene is captured by an array ofoptical sensors such as detector 190 (e.g., a CCD semiconductor chip).Such an interference space may be transparent and may comprise a vacuum,air, a gas other than air, a liquid or a solid (e.g., a lens or a slabwaveguide).

Given that the positions of the individual antenna elements 120 in thearray 110 are fixed, the phase relations of waves sampled by theseelements depend on both the angle of arrival and on the frequency. Forexample, in a system where the geometry of the lenslet array 150 matchesthe geometry of the antenna array 110, two waves arriving at the RFaperture from the same direction but differing in frequency will(normally) reconstruct in the optical domain as bright spots indifferent positions on the image plane (e.g., on photodetector 190 fordetection and processing by processor 200), as shown in FIG. 3A. Theamount of spatial offset between different RF waves with differentfrequencies incident upon the array depends on the incidence angle: forwaves arriving at the array along the RF imaging axis, or achromaticaxis (which may be considered an incidence angle equal to zero), all RFfrequencies reconstruct to a single spot lying on the optical axis ofthe imaging system. The greater the incidence angle of the RF wave withrespect to the RF imaging axis, the greater the spread of the resultingoptical image as a function frequency. Using the terminology from thefield of imaging optics, such spreading of an image due to change infrequency (wavelength) is referred to herein as chromatic aberration.

The effect of chromatic aberration in the imaging receiver withhomothetic arrays 110 and 150 is illustrated in FIG. 3A. It will beappreciated that the optical reconstruction referenced below (e.g.,detection of optical spots) may be performed by an imaging receiver,such as the imaging receiver 100 described herein. The opticalreconstruction may be captured in real time by detector 190 of theimaging receiver 100. For example, the optical dots discussed herein maybe detected by detector 190 and processed by processor 200. For suchoptical reconstruction, the imaging receiver may use a single detector190 detecting light of a single composite beam 185 formed from one ormore optical fiber bundles (where the outputs of the plural opticalbundles described herein are combined), or in certain examples, use asingle antenna array 120 that has outputs of separate optical fiberbundles to different ones of plural detectors 190, where each detector190 is associated with separate optical processing elements describedhere, and each detector 190 is associated with a separate optical fiberbundle. FIG. 3A illustrates incoming RF radiation at frequencies Ω₁ andΩ₂ incident upon the array. The angle of arrival is identical for thetwo RF beams, but the frequencies (wavelengths) differ. Reconstructed inoptical domain, two spatially-separated spots are formed: onecorresponding to incoming frequency Ω₁ and the other to Ω₂.

Chromatic aberration can also be understood with the help of wave-vector(or k-space) description of the image reconstruction. An RF plane waveincident upon the antenna array is represented by a wave vector (ak-vector) pointing in a direction perpendicular to the phase-front andhaving length proportional to the frequency; the k-vector points in thedirection of propagation of the wave. The imaging receiver, by virtue ofupconverting the RF waves to the optical domain followed by opticalreconstruction of the RF waves (e.g., by photodetector 190), may map thek-vectors of the incoming RF waves onto the image plane of the opticallens, see FIG. 3B. The k-vectors corresponding to all possible RF planewaves form a three-dimensional vector space. This 3D space is mappedonto a two-dimensional space: the image plane of the imaging lens, whichmay correspond to the 2D plane of the photodetector 190 when implementedas an image sensor. The 3D to 2D mapping may be a projection along theachromatic axis of the imaging receiver 100. As a result, k-vectorsdiffering by a vector parallel to the achromatic axis of the imagingreceiver 100 are mapped to the same point in the image plane. Thissituation is shown in FIG. 3C where plane waves corresponding to vectorsk₁, k₂, and k₃ are all mapped to a single point in the image plane asthey differ from one another by a vector parallel to the achromaticaxis. In contrast, wave-vectors k₄ and k₅ are mapped to two separatepoints in the image plane even though they are parallel to one another,i.e. they correspond to plane waves coming from the same direction. Thedifference in length between wave-vectors k₄ and k₅ is due to thedifference in frequency of the underlying RF waves.

In short, the imaging receiver maps the 3D space of wave-vectors to a 2Dimage plane by projecting the former along the achromatic axis. Thisleads to chromatic aberration where some waves arriving from the samedirection are mapped to different points (e.g. wave-vectors k₄ and k₅ inFIG. 3C), and certain waves arriving from different directions map tothe same point (e.g. wave-vectors k₁, k₂, and k₃ in FIG. 3C).

The above statement can be understood with the help of FIG. 4, whichshows a portion of k-space. In this representation, every point is ak-vector (wave-vector) that corresponds to a plane wave arriving at thereceiver. The length of the k-vector (the distance of the point fromorigin located at the center of the semicircles in FIG. 4) isproportional to the frequency and the angle of arrival of the wave isthe vector's direction. Given this one-to-one correspondence betweenwaves arriving at the receiver and the points in k-space, the latter ishelpful when describing the imaging receiver.

Accordingly, since the imaging receiver performs a projection along theachromatic axis, in k-space this projection takes a geometric meaning:points along each of the lines labeled as “Lines of constant k_(x)” inFIG. 4, are represented as a single point in the imaging receiver inthis example. FIG. 4 illustrates just five lines of constant k_(x) forsimplicity. The above perspective on the imaging receiver provides meansto generalizing the concept and enabling access to information that iscaptured by the distributed array. The imaging receiver may includestructure to implement one or more of the following features:

-   -   Sampling the incoming electromagnetic field at discrete points        using an array of antennas (120).    -   Upconversion of the received electromagnetic radiation to        optical domain at each of the sampled points. This is        accomplished with electro-optic modulators (130).    -   Conveying the upconverted signals, now in optical domain, using        optical fibers (140), one or more fibers per antenna, to a fiber        array.    -   Using fiber array that may be a scaled version of the antenna        array.    -   Free-space propagation (160) and optical processing of light        emanating from the output of the fiber array, which contains the        information on the received electromagnetic radiation.    -   Optical capture of the RF scene: The interference pattern within        composite beam 185, at every received RF frequency, may        correspond to the RF scene observed by the antenna array. The        optical capture of the RF scene may be processed to reconstruct        the RF scene.

As described herein, information about radio frequency (RF) emittersfrom received electromagnetic radiation may be extracted. The exemplaryimplementations may provide real-time, simultaneous determination ofcarrier frequency, amplitude and angle of arrival (AoA). In someembodiments, instantaneous bandwidth (IBW) may approach 100 GHz. Thiscapability may be achieved without sacrifice of signal-to-noise ratio(SNR), by virtue of an antenna array whose gain more than compensatesfor the added thermal noise that accompanies such wide IBW. The opticalapproach may enable the array's entire field of regard (i.e. its fullbeam steering range) to be continuously detected and processed in realtime.

Optical image formation and engineered spectral dispersion may be usedto acquire multiple k-space projections of the RF scene. Opticalupconversions of RF signals by high performance modulators enables theuse of simple, inexpensive optical components to perform correlationsamong the signals received by the array elements. For the IBW andresolution (in both frequency and AoA) provided with this approach, suchcorrelations would be intractable using a conventional approach based ondownconversion, channelization, A/D conversion and computationalcorrelation. For example, 8-bit digitization of 100 spectral channels,each 100-MHz wide and together spanning 10 GHz, for 1000 simultaneousspatial directions (array beams) requires 20 TB/s of data throughput,not to mention the computational burden of analyzing all that data inreal time, nor the sheer size and scale of 1000 parallel channelizedreceivers.

The optical approach may include the following: RF signals are receivedby antennas 120 that feed modulators 130, which upconvert the signalsonto optical carriers conveyed by fibers 140. The sidebands are launchedinto free space as a composite optical beam 160 through an output fiberbundle that replicates the arrangement of antennas 120 in the array, atreduced scale. In this way the optical output of the bundle comprises ascaled replica of the RF field incident on the antenna array aperture.In some embodiments, the output of the fiber bundle need not replicatethe arrangement of the antennas 120. Simple optical lenses and a camera(focal plane array of detectors 190) can then be used to capture theinterference pattern of the composite optical beam 160, from which anoptical image of the RF scene may be obtained (i.e. a map of the AoA andamplitude of any and all RF emitters sensed by the antenna array 120).The optical image of the RF scene may be obtained with straightforwardcomputational processing. To add frequency determination to this imagingcapability, the lengths of the output fibers are made unequal, so as tointroduce a controlled chromatic dispersion (e.g. linearly ramping thelength across the array, which is effectively an RF diffraction grating,or implementing lengths that have no correlation (e.g., may be randomlengths) across the array), spreading the frequency content of thesignals out in the image seen at the camera. Alternatively, or incombination with making the lengths of the optical fibers unequal, thespreading of the frequency content may be achieved by distributing theantennas in a non-coplanar configuration. This spreading of thefrequencies mixes the spatial and spectral information about a signal inthe image. The modulator outputs may be split into multiple fibers, andmultiple output fiber bundles may be used to form multiple images. Eachoutput fiber bundle may contain a different distribution of the fiberlengths, by which each corresponding image represents a differentprojection of the full spatial-spectral scene.

The most appropriate conceptual framework for understanding this processis k-space. Every RF signal incident on the array can be characterizedby a wavevector k, also called a k-vector. K-space is just a uniformequivalent of an abstract space comprised of up to 2 dimensions of AoA(azimuth and elevation) and 1 dimension of (temporal) frequency.Recalling that the magnitude of the wavevector is directly related tofrequency according to 2nf=ck, one can readily see that frequency andAoA represent a set of spherical coordinates spanning k-space. Thinkingin terms of wavevectors, rather than AoA and frequency, we are free toanalyze the scene using other coordinate systems, e.g. Cartesian:{k_(x), k_(y), k_(z)}. Each of the multiple images can be interpreted asa different projection of the full k-space. For example, when all fiberlengths are equal, this corresponds to a projection onto the aperture(x-y) plane, which is insensitive to k_(z), as shown in FIG. 4.Variation of the lengths provides different projections. As intomography in real (position) space, which builds 3D images of theinterior of structures by combining multiple projections, computationalreconstruction techniques can be used to build the full k-spacedistribution of RF emitters from the multiple projections. From thisk-space “scene,” the frequency and AoA of each individual emitter can beextracted.

Analysis and simulations show that with this approach, received signals'carrier frequency can be determined to ≈100 MHz or better, depending onthe variation of the lengths of the fibers and signal-to-noise ratio,and this can be accomplished simultaneously for multiple signals atwidely disparate frequencies, while simultaneously providing AoA aswell. The precision of the AoA determination depends on the ratio of thecarrier wavelength to the overall aperture size, as well as SNR: as anexample, in the low-noise limit, <1° accuracy can be obtained with a6-cm array aperture at 18 GHz.

Generalization of Imaging Receiver

The disclosed imaging receiver may be in accordance with one or more ofthe following features:

-   -   Allowing variation of the fiber length amongst the different        optical channels.    -   Allowing multiple optical fibers per antenna.    -   Allowing arbitrary geometry of the fiber array, not necessarily        linked to the geometry of the antenna array.    -   Allowing the geometry of the antenna array not to be flat (2D);        antennas in the array may be distributed in three dimensions,        for example following a contour of a curved surface.    -   Computational reconstruction of the RF scene that includes        extracting both the angle of arrival and frequency of the        incoming RF radiation.

The interference pattern produced by light emanating from the opticalfibers may no longer correspond directly to the RF scene. Instead, thefollowing general relation holds between the RF sources and the detectedoptical powersP _(n) =a _(n) ·S  (1)

where a_(n) is a (abstract) vector corresponding to the n-th opticaldetector, S is a (abstract) vector corresponding to the distribution ofsources in the k-space, i.e. the RF scene, and P_(n) is the powerdetected by the n-th detector.

Expression (1) can be manipulated to obtain the following equivalentforms

$\begin{matrix}{{P_{n} = {\sum\limits_{m}{a_{n\; m}S_{m}}}}{P = {AS}}} & (2)\end{matrix}$

where the first of Eqs. (2) explicitly shows the summation of the dotproduct in Eq. (1) whereas the second of Eqs. (2) shows a compactnotation involving matrix multiplication of (sought) vector S by matrixA to obtain the measured vector P of detected optical intensities. InEq. (2), matrix A is determined by the details of the imaging receiverthat include the geometry of the antenna array, the geometry of thefiber array, and the lengths of the fibers, as well as any additionaloptical phases applied to the optical signals conveyed by the opticalchannels. Vector S describes the RF scene in k-space, i.e. thefrequencies (or frequency distributions), angles of arrival andintensities of the RF sources whose signals are received by the antennaarray. Vector P comprises the intensities measured by thephotodetectors. Hence, the reconstruction of the RF scene based ondetected (measured) optical intensities P may require the ‘inversion’ ofthe relation Eq. (2). Since matrix A may in general be rectangular (notsquare) and/or singular, such ‘inversion’ may not be well defined ingeneral. In this case, an approximate, and ‘most likely’ vector S issought that satisfies Eqs. (2) or Eq. (1). Note also that in Eq. (2),finding the left inverse of matrix A would be sufficient to reconstructthe scene.

There exist a variety of methods that can be used to find S thatsatisfies, or approximately satisfies, Eqs. (2) or Eq. (1) givenmeasured/detected P. For example, methods used in computed tomographymay be employed that include the algebraic reconstruction technique(ART) also known as Kaczmarz method, or its multiplicative version(MART), or their more sophisticated flavors known to those skilled inthe art. Such methods may maximize the entropy, or the relative entropy,or Kulback-Leibler divergence, of the reconstructed RF scene, or, inother words, may find the most likely distribution of RF sources(frequencies, intensities and angles of arrival of the received waves)that would result in the detected values of P. Also, ‘inverting’ arelation akin to that of Eq. (2) is encountered in compressive-sensingreconstruction. Therefore, methods used in that field may be applicablehere.

To facilitate and speed the reconstruction of RF scenes, a look-up tablecan be constructed by, for example, direct sensing of known scenes whichcan be augmented by computational processing using known tomographytechniques, as described above, on selected matrix entries as necessary.The look-up table may receive inputs comprising one or more pixelcoordinates corresponding to the location(s) of detected light by thephotodetector 190. Based on receiving these pixel coordinate inputs, thelook-up table may output one or more k-space vectors, each k-spacevector identifying the frequency and AoA of a corresponding RF source ofthe RF scene. In some examples, the look-up table may also receiveinput(s) of the intensity of the detected light by the photodetectorcorresponding to each of the one or more pixel coordinates and outputk-space vector(s) based on such intensity input(s).

FIG. 6 shows an example of RF scene reconstruction using a linear arraywith randomized antenna position and a random distribution of fiberlengths. On the left is the original (input) distribution of four RFsources present in the scene as represented in k-space. On the right isthe reconstruction of the scene by inverting relation (2). Excellentreconstruction fidelity is accomplished in this case.

FIG. 7 shows the distribution of baselines used in the reconstruction ofFIG. 6. The baselines are provided by the fiber-length differences (Δt)and by the separations in the x-direction (Δx) of the antennas in thearray.

The above describes the general mode of operation of the cuing receiver.There may be other modes of operation that may relax the computationalburden of extracting the information about the RF scene. Below, examplesof some of such modes of operation are described in some detail.

Homothetic Arrays

FIG. 2D illustrates details of an embodiment where the detector 190 maycapture in real-time an image (still image and/or video image) of aselected frequency of the RF scene with non-selected frequencies beingeffectively filtered and treated as noise by the detector 190. In thisexample, one fiber per antenna may be used and the geometry of the fiberarray bundle at lenslet 150 (at the ends 141 of the optical fibers 140)may be a scaled version of the antenna array 120. For example, the ends141 of the optical fibers 140 may have the same relative physicalarrangement as the arrangement of the antenna array 120. For example, aprojection of the antenna array 120 onto a plane may have the samerelative arrangement as the arrangement of the ends 141 of opticalfibers 140 corresponding to the connection of such optical fibers 140 tothe associated antenna array 120. The same relative arrangement mayinclude the same relative spacing, same relative order and/or samerelative locations with respect to neighboring fiber ends 141.

As shown in FIG. 2D, a phase offset 204 is applied to optical modulator130 by applying a constant (DC) bias voltage; to obtain optical phasedelay ϕ, voltage V=(ϕ/π)*V_(π) is applied, where V_(π) is the half-wavevoltage of the electro-optic modulator. The phase offset 204 is variableand is based on a selected frequency 202 input to the processor 200 andon the (optical) length of the optical fiber 140 a. The processor 200outputs an appropriate phase offset 204 for each of the modulators 130of the imaging receiver 100 to compensate for the phase delay the RFsignal would experience when traversing the distance L_(140a), which isequal to ω_(m)*L_(140a)/c, where ω_(m) is the selected RF frequency 202,L_(140a) is the optical length of the fiber 140 a (time delay multipliedby the speed of light) and c is the speed of light. Note that theapplied optical phase offset to cancel the accrued RF phase delay needonly be applied modulo 2π. Since the phase delay, ω_(m)*L_(140a)/c, ofthe RF signal is an explicit function of the selected frequency 202,ω_(m), the applied optical phase compensation provides phasecancellation only for that selected RF frequency. Similarly, differentoptical lengths L_(140a) require different optical phase compensations.

With these phase offsets applied to each of the modulators 130, despitethe different lengths of the optical fibers 140, the upcoverted opticalsignals corresponding to the selected RF frequency remain in properphase relations at the outputs 141 of the optical fibers 140 for theoptical interference (e.g., constructive and destructive interference)in the composite beams 160 and 185 to reproduce the RF scene at thisselected RF frequency as an optical image on the detector 190 (as stilland/or video image). This way, the image projected onto and detected bythe detector 190 corresponds to the RF scene of the selected RFfrequency received by antenna array 120. However, optical signalscorresponding to RF frequencies outside this selected RF frequency willexit the optical fiber bundle without compensation for the phasedifferences caused by the different lengths of the optical fibers 140 inthe optical fiber bundle and thus will be distributed across thedetector 190 and appear as noise to the detector 190.

With the phase compensation as described with respect to FIG. 2D, the RFscene at the selected RF frequency is faithfully reconstructed in theoptical domain, i.e., the interference pattern generated by theoverlapping optical beams emanating from the fiber array corresponds tothe RF scene at the selected frequency, plus distributed background(fixed-pattern ‘noise’) due to sources operating at other frequencies.As such, little or no additional (computational) processing is needed todetermine the angle of arrival of waves at this frequency. Sourcesoperating at frequencies different than the selected one contribute tothe detected power, but their contribution is, in general, spread overmultiple detectors. As a result, the contribution of such sources to thesignal at any selected photodetector corresponding to a particular angleof arrival would be suppressed as compared to the frequency the receiveris ‘tuned to.’ Such contribution from out-of-band sources may be furthersuppressed by applying spectral filtering at the RF front end, i.e.before the up-conversion of the received RF signals to optical domain.Also or alternatively, optical filtering to suppress the contributionfrom out-of-band sources may be used.

In the exemplary implementation above, optical fibers with varyinglengths were utilized however other means for effecting phase variationcan be utilized. For example, true time delay lines—either adjustable orfixed—can be utilized to introduce length variation. Adjustable timedelay provides the benefit of adjusting or fine tuning system operationon the fly.

The selected RF frequency 202 at which faithful optical reconstructiontakes place (detected as an image by detector 190) may be selected by auser based on certain RF frequencies of interest and/or automaticallychanged rapidly by applying suitable bias or phase offset to themodulators. Hence, the received frequency may be scanned to reconstructthe distribution of RF sources in the k-space, i.e. finding thefrequencies, intensities and angles of arrival of the receivedelectromagnetic waves.

Multiple Independent k-Space Projections

As indicated in FIG. 4, equal fiber lengths in combination withhomothetic arrays yield a particularly simple projection of the k-spacein the optical reconstruction. On the other hand, purely randomselection of fiber lengths yields projections of the type indicated inFIG. 5. By choosing the fiber lengths to vary linearly with the positionof the antenna in the array, and homothetic arrays configuration, theprojection of the k-space that deviates from that of FIG. 4, but onethat is less complex than that of FIG. 5 may be obtained. Such acuing-receiver configuration is shown in FIG. 8. One can think of suchconfiguration as receiving all incoming RF radiation on one side of theachromatic axis.

As an example, in FIG. 8, the optical-fiber length corresponding to theantenna element at the top of the array is shorter than the opticalfiber-fiber length corresponding to the element at the bottom. (It isnoted that the fiber-length differences in FIG. 8 are shown forillustration purposes only, and may be considerably different in anactual implementation of the system.) As a result, an RF wave incomingalong the line labeled in FIG. 8 as “Achromatic axis” would produce anoptical wave-front parallel to the optical axis of the imaging lens, hadit not been outside the field of view. On the other hand, the wavelabeled as “incoming RF” would produce a spot off axis as shown in thefigure.

The manifestation of such receiver configuration in k-space is shown inFIG. 9. Compared to the configuration of equal-length fibers of FIG. 4,the projection lines are tilted in the k-space. As a result, mixing ofthe angle of arrival and frequencies occurs in the light distributiondetected by the photodetector array. It is also possible to configurethe array in such a way that the RF imaging axis (achromatic axis) fallsoutside the field of view of the antenna elements as shown in FIG. 8.Note that the field of view is determined by the acceptance angle of theindividual elements of the antenna array.

Combining two receivers with different RF imaging axes, as in FIG. 10,yields two images of the same RF scene with differentfrequency-dependent shift in the optical reconstruction: Note that forthe same two RF sources operating at frequencies Ω₁ and Ω₂, the image ofsource 1 is shifted down with respect to source 2 in imager with RF axisA, whereas it is shifted up in the imager with RF axis B.

In an abstract sense, the use of two arrays with different RF imagingaxes can be visualized with the help of FIG. 11A. The 3D space ofpropagation vectors k is projected onto two dimensions along axes A andB corresponding to the two arrays. Accordingly, two 2D images are formedin the optical domain with the 3D k-vector corresponding to an incomingRF wave represented as a 2D vector in each of the arrays. Having the two2D projections, the original 3D k-vector can be reconstructedcomputationally. This way, full information of the incoming wave, i.e.the AoA and the frequency, can be recovered from the two 2D images.

The reconstruction of the 3D k-vector from two 2D projections can becompared to 3D real-space stereoscopic imaging, FIG. 11B. In that case,the image projected on the retina of each eye is two-dimensional, yet a3D representation of the scene is reconstructed ‘computationally’ in thebrain from two such images obtained by two projections along two opticalaxes of the left and right eyes. Similar computational reconstruction ofthe 3D k-vector space is performed in exemplary implementations of ourinvention.

FIG. 13 is another illustration of the two projections of k-space. Twosets of projection lines are present, each set corresponding to one ofthe achromatic axes, see FIG. 12.

Although the 3D k-space reconstruction of the exemplary implementationsare conceptually similar to the stereoscopic vision described above,there may be differences between these two cases. Whereas stereoscopicvision applies to imaging objects in real space, our system may apply tok-space—the space of k-vectors corresponding to plane waves. As aresult, for the stereoscopic vision to be effective, the two parts ofthe imaging system must be offset spatially, as in the familiarLeft-eye/Right-eye configuration of FIG. 11B; this is how the imagingaxes are made non-parallel, and each eye presents a different view ofthe same subject. In contrast, since the imaging receiver performsprojection in k-space, the two antenna arrays of the imaging receiver inFIG. 10 can be co-located as long as they present different imaging axesby, for example, properly choosing the optical fiber lengths.

The ability to co-locate the two arrays can be taken advantage byinterleaving the antenna placement as in FIG. 12. This configuration canbe thought of as consisting of a single antenna array with fiberscollected in two separate bundles to form two optical images; each ofthe fiber bundles carries RF signals from antenna elements scatteredthroughout the array. The fiber lengths for each of the bundles arechosen so as to yield RF axes A and B, FIG. 12, similar to the RF axesof the two spatially-separated arrays of FIG. 10.

The idea of co-locating the two arrays can be implemented as shown inFIG. 14 where a single antenna array is present. The modulated opticaloutput of each antenna element is split into two, and the resulting twosets of fibers are collected into two fiber bundles to reconstruct twooptical images. The lengths of the fibers in each of the bundles arechosen so as to produce different RF optical axes, labeled as RF axis Aand RF axis B in FIG. 14. As a result of having originated from twodifferent RF imaging axes, the two optical images correspond to twodifferent projections of the 3D k-space, as explained in FIG. 11A, andtherefore provide means to reconstructing the AoA and frequency ofreceived RF waves.

For imaging receivers configured in such a way that the RF imaging axesfall outside the field of view of individual antennas, the system can befurther simplified by combining optical reconstruction. Since thek-vector of the received incoming RF wave always falls to one side ofthe RF imaging axis, the resulting optical image will fall to one sideof the optical axis. As a result, the two optical images form on theopposite sides of the optical axis. This allows combining the twooptical systems into one where half of the image corresponds to theprojection of the k-space along RF axis A, and the other halfcorresponds to the projection of the k-space along RF axis B.

The disclosed exemplary implementations may resolve the ambiguity in theangle of arrival (AoA) of electromagnetic radiation at the position of adistributed aperture. In addition, it provides information about thefrequency of received radiation. The exemplary implementations may do sowithin the framework of imaging receiver concept wherein the incoming RFis up-converted to optical domain in the front end, i.e. at theindividual antenna elements that constitute the receiving array, andconveyed with optical fibers to central location for processing.

The exemplary implementations allow the use of relatively slowphoto-detector array for detector 190, one that need not respond at theRF frequencies received, to extract information about the RF sourcelocation (AoA) and the transmitted frequency. Although the disclosedexemplary implementations will also operate with the use of a relativelyfast photo-detector array for detector 190, it is not necessary and mayconstitute a wasted extra expense. As described herein, each of theplural fiber optic bundles may have their optical outputs 141 imaged bythe same detector 190 simultaneously or may each have their opticaloutputs 141 imaged by a different detector 190. In addition, the pluraloptical bundles may have their optical 141 outputs imaged separately andsequentially (e.g., rapidly switch the optical outputs 141 of eachoptical bundle on and off to detect the optical outputs at differenttimes by detector 139). This latter implementation may be helpful whenattempting to resolve ambiguities in analysis of certain opticalpatterns detected by detector 190 of the combined optical outputs 141 ofplural fiber optic bundles onto detector 190.

To the best of our knowledge, prior to this invention, there was no wayto resolve the AoA ambiguity within the imaging-receiver framework,absent post-detection (using fast photo-detector(s)) electronicprocessing to determine the incoming frequency.

The exemplary implementations may unambiguously pin-point the locationand frequency of an RF source with the imaging receiver.

The implementation of the general imaging receiver may require intensivecomputation to achieve faithful reconstruction of the RF scene. On theother hand, the use of ‘stereoscopic’ reconstruction with multipleprojections of the k-space may in some circumstances lead to ambiguousreconstruction. For example, if multiple RF sources are transmittingsimultaneously in the scene, there remains the possibility of assigningincorrect AoA and frequency in some circumstances. Such possibility canbe related to the stereoscopic vision, compare FIG. 11B, which in mostcases is sufficient to faithfully reconstruct a 3D scene, but allowsoptical illusions in some circumstances, with false impression of objectplacement and/or orientation in the scene.

This disadvantage can be overcome by increasing the number of arrayssimultaneously trained on the scene (e.g., three or four or morearrays), with each array presenting a different RF imaging axis. Eachadditional array introduces additional constraints on the coincidentalplacement of RF sources and their frequencies that could lead toambiguity. Therefore, each additional array reduces the likelihood ofsuch a coincidence happening. Such arrangement can be compared totomography where a series of projections at various angles, acomputer-tomographic (CT) scan, allows the 3D reconstruction in realspace. Since our invention relates to k-space, such reconstruction canbe referred to as k-space tomography in our case. The reduced ambiguityis at the cost of increased computational complexity to reconstruct theRF scene.

FIG. 16 is a flow chart of a method performed by an imaging receiverconfigured in accordance with the disclosed exemplary implementationsand FIGS. 1, 2A-C. In S1600, sampling of the RF signal field received atdistributed antenna arrays is performed. In 51610, the sampled RFsignals are modulated onto optical beams, and in S1620 the modulatedsignals are conveyed by, for example, optical fibers having varyinglengths in accordance with S1630 so as to generate interference patternsamongst the modulated optical beams in S1640. The optical beaminterference is recorded by photodetectors in S1650 and computationalreconstruction of the RF signals in k-space is performed in S1660 usingknown techniques.

One way of providing the different path lengths, per S1630 in FIG. 16,is by using optical fibers having varying lengths carrying the modulatedoptical beams to the photodetectors for each optical beam. In oneexemplary implementation, the length of the fibers varies linearly inaccordance with its position at the antenna/modulator array. Analternative methodology would be to use different configurations for thefiber array as compared to the antenna array as, for example, shown inFIG. 15. Another alternative would be to use arbitrary, e.g. random,fiber lengths as illustrated in FIG. 5. The range of fiber lengths mayaffect the spectral resolution of the obtained reconstruction of the RFscene. Thus, the spectral resolution may be determined by the largestdifference in fiber length in accordance with well-known scientificprinciples. For example, if the largest fiber length difference leads tothe relative delay between respective optical signals of 1 ns, then thespectral resolution, i.e. the ability of the system to distinctlyresolve RF sources emitting at adjacent frequencies, may be about 2*(1ns)⁻¹=2 GHz. The practical path length variations implemented in opticalfiber may range between 0.5 mm and hundreds of meters.

It will be apparent to those of ordinary skill in the art that theinventive concepts described herein are not limited to the aboveexemplary implementations and the appended drawings and various changesin form and details may be made therein without departing from thespirit and scope of the following claims.

What is claimed:
 1. An optical imaging receiver comprising: aphased-array antenna including a plurality of antenna elements arrangedin a first pattern configured to receive RF signals from at least one RFsource; a plurality of electro-optic modulators corresponding to theplurality of antenna elements, each modulator configured to modulate anoptical carrier with a received RF signal to generate a plurality ofmodulated optical signals; a plurality of optical channels configured tocarry the plurality of modulated optical signals and configured to causeinterference amongst the optical signals, each of the plurality ofoptical channels having an output to emanate the corresponding modulatedoptical signal out of the corresponding optical channel, the outputs ofthe plurality of optical channels arranged in a second pattern whichdoes not correspond to the first pattern; a plurality of photodetectorsfor recording the optical signal interference; and a module forcomputationally reconstructing RF sources in k-space from the recordedinterference.
 2. The optical imaging receiver of claim 1, said pluralityof optical channels comprising a plurality of optical fibers, theoptical fibers having varying lengths.
 3. The optical imaging receiverof claim 2, wherein at least two optical fibers of said plurality ofoptical fibers are connected to the output of at least one of theplurality of modulators.
 4. The optical imaging receiver of claim 1,wherein the distribution of antennas in the phased-array antenna isnon-coplanar.
 5. The optical imaging receiver of claim 1, wherein themodule uses a computational tomography technique which include algebraicreconstruction technique (ART) or its multiplicative version (MART) inreconstructing the RF sources in k-space from the recorded interference.6. The optical imaging receiver of claim 2, wherein lengths of theoptical fibers vary linearly in accordance with their position inrelation to the antenna array.
 7. A method utilized by an opticalimaging receiver for RF signal processing, comprising: receivingincoming RF signals at a phased-array antenna including a plurality ofantenna elements arranged in a first pattern; modulating the received RFsignals from each of the plurality of antenna elements onto an opticalcarrier to generate a plurality of modulated optical signals; directingthe plurality of modulated optical signals to a plurality of opticalchannels, configured to cause interference amongst the optical signals,each of the plurality of optical channels having an output to emanatethe corresponding modulated optical signal out of the correspondingoptical channel, the outputs of the plurality of optical channelsarranged in a second pattern which does not correspond to the firstpattern; providing a plurality of photodetectors for recording theoptical signal interference; and computationally reconstructing RFsources in k-space from the recorded interference.
 8. The method ofclaim 7, said plurality of optical channels comprising a plurality ofoptical fibers, the optical fibers having varying lengths.
 9. The methodof claim 8, wherein at least two optical fibers having varying lengthsof said plurality of optical fibers are connected to the output of atleast one of said plurality of modulators.
 10. The method of claim 7,wherein the distribution of antennas in the phased-array antenna isnon-coplanar.
 11. The method of claim 7, wherein computationallyreconstructing involves using a technique which includes algebraicreconstruction technique (ART) or its multiplicative version (MART) forreconstructing the RF sources in k-space from the recorded interference.12. The method of claim 8, wherein the lengths of the optical fibersvary linearly in in accordance with their position in relation to theantenna array.